There are inkjet recording apparatuses operable as an image forming apparatus for propelling droplets of ink out of nozzles to thereby draw images. Such inkjet recording apparatuses sometimes experience ink droplets being discharged in deviated directions, or in some cases even a failure to discharge ink droplets. Deviated discharge directions make flexed orbits, resulting in deviated shot positions on a printing medium. Such phenomena may be caused by a degraded precision in production of nozzles used for propelling out ink droplets, or by paper dust or such adhering to discharge ends of nozzles. Occurrences of such phenomena may produce white lines in an image forming region on a printing medium, causing the image quality to be degraded.
In particular, white lines produced in the bar code printing, which requires a high precision of printing, may change the width of black bar or separate black bars, causing a reading device such as a scanner to fail in reading a correct bar code.
Against such an issue, there are techniques including detecting nozzles failing to discharge ink, and printing those images which should have been formed by the discharge failing nozzles (Patent Literature 1: Japanese Patent Publication No. 3157880, Patent Literature 2: Japanese Patent Application Laid-Open Publication No. 2-22066, Patent Literature 3: Japanese Patent Publication No. 2989723, Patent Literature 4: Japanese Patent Publication No. 3313819, and Patent Literature 5: Japanese Patent Application Laid-Open Publication No. 4-185462). More specifically, Patent Literatures 1 and 2 have disclosed techniques including detecting nozzles failing to discharge ink, and using nozzles in other ink heads to complement those regions located under the discharge failing nozzles. Patent Literatures 3 and 4 have disclosed techniques including detecting nozzles failing to discharge ink, using other nozzles in the same ink head to complement the regions under the discharge failing nozzles. Patent Literature 5 has disclosed techniques employing a multi-pass system in which ink heads discharge ink, moving perpendicularly to the transfer direction of a printing medium, and repeat a plurality of passes commensurate with the scan width. This technique includes detecting nozzles failing to discharge ink, and complementing the regions under the discharge failing nodes in other passes.
However, the techniques disclosed in Patent Literatures 1 to 5 need detecting all nozzles for problematic ink discharge. This detection needs additional provision of detecting elements such as optical sensors, causing the equipment cost to be increased. In particular, those single pass systems using a line head longer than the width of a printing range require a length of optical sensor array commensurate with the printable width. This requirement invites a wide increase in production cost. Further, those configurations providing an ink head to cover missing dots like Patent Literature 2 have an increased cost for addition of the head. Also those configurations arraying nodes in parallel to the sheet transfer direction like Patent Literature 4 push up the production cost of the head itself.
Some methods use nozzles working for discharge to complement the regions under discharge failing nozzles, assuming a configuration specific to a printing system such as a single pass system or a multi-pass system. Such methods are inapplicable to different printing systems.
More specifically, there are inkjet recording apparatuses categorized into a group of single pass systems or a group of multi-pass systems, as described. Single pass systems employ a line head longer than the width of a printing range, and form all regions to be printed for a printing while a printing medium once passes under the line head. Inkjet recording apparatuses using single pass systems are thus given no more than a single chance to have a recording medium passing under a line head. Therefore, they are unable to employ any system based on a premise of passing printing regions a plurality of times for a printing thereon, affording to make use of different passes for a printing to complement the regions under discharge-failing nozzles.
Likewise, dedicated units for the monochrome printing are in applicable to those methods based on a configuration specific to the color printing, to use nozzles working for discharge to complement the regions under discharge failing nozzles. That is, monochrome printers simply provided with an ink head for a black color are unable to employ any system using a head for a different color for the complementation, as disclosed in Patent Literature 1, for instance.
It is noted that Patent Literature 6 (Japanese Patent Application Laid-Open Publication No. 2006-82528) has disclosed a technique not based on any premise of detecting nozzles for failed discharge. The technique disclosed in Patent Literature 6 employs an n-row by n-column matrix of nozzles (n: two or larger integer) for discharging ink to a single pixel, and uses nozzles working for discharge to complement the regions under discharge failing nozzles.
However, the technique disclosed in Patent Literature 6 must discharge ink from two or more nozzles to form a single pixel. There may be a case needing a specification for the configuration exceeding a required resolution. It therefore is difficult to take this technique as a versatile solution.